The invention is in the field of radiography and addresses the problem that variations in tissue thickness within the anatomy (or of object thickness in the case of inanimate bodies) can cause large variations in exposure at the image plane that can exceed the practical or desirable exposure range of the film or other imaging medium.
Various efforts have been made in the past to address this problem, in principle by seeking to reduce the range of exposure as between different areas of the image. For example, Old Delft U.S. Pat. Nos. 4,675,893, 4,677,652, 4,715,056 and 4,741,012 propose sWeeping the imaged object with a fan-shaped beam of x-rays, detecting the post-patient intensity of sectors of the beam and using a feedback scheme to modulate the pre-patient intensity of sectors of the fan beam in a way which would reduce image plane variations in exposure. To this end, these patents propose the use of attenuating reeds which are moved to a greater or lesser degree into the respective sectors of the beam under servo control. The intent is to attenuate a sector to a greater or lesser degree, but it is believed that the intent is not to totally turn off a sector of radiation. A proposal which is believed similar in principle is made in Edholm U.S. Pat. No. 3,755,672, and involves using a feedback scheme to form a radiation filter which would attenuate a conical or pyramidal beam to different degrees depending on the solid angle position in the beam.
Other techniques of beam equalization are discussed in said parent patent application, including the sweeping of the sectors of a fan beam of x-rays across the object while individually controlling the respective velocities of the sectors as functions of the post-object intensities of the sectors or, in the alternative, pulsing the sectors in a pulse-width modulation scheme in which the pulse widths depend on measurements of post-object intensities of the sectors.
The techniques proposed in the Old Delft patents cited above appear to suffer from the need for precise dynamic positioning of the reeds while the fan beam sweeps the patient and for precise calibration to ensure that at any one time each reed attenuates its sector of the fan beam to the required degree, and also from the need to minimize the inherent lag between the detection of a need to attenuate to a certain degree and the movement of a reed to the precise position which could provide the needed attenuation. Similar considerations appear to apply to the Edholm patent cited above, as well the additional consideration of the need to form the filter before the imaging exposure could start. In addition, it is believed that there may be time variations in the output of the x-ray tube which may be too rapid to be accommodated by system of the type proposed in the Old Delft and Edholm patents. The techniques taught in said parent application provide effective equalization for improved imaging but a need for improvement still remains. For example, the systems described in said parent application which scan a single, pencil-shaped x-ray beam in raster fashion need a significant amount of time to complete the scan or would need high x-ray output to speed up the scan. The systems which sweep the sectors of a fan beam while individually modulating the sectors through velocity modulation or pulse width modulation are described as implemented through relatively complex mechanisms for effecting the sector modulation.
Accordingly, this invention is directed to overcoming these and other limitations and to providing an imaging system in which a fan beam of penetrating radiation is swept across the object which is to be imaged while individually pulse width modulating sectors thereof as a function of post-object radiation, using a highly effective and practical feedback scheme. The object can be a patient or an inanimate object such as an industrial part. The modulation can be carried out by using bistable shutter pins. Each shutter pin has only two stable positions and transits between them rapidly under feedback control. In a blocking position a shutter pin completely blocks its sector of the fan beam and in an open position the pin completely uncovers its sector of the fan beam. The sweep of the fan beam across the object being imaged is divided into a number of sampling time intervals and each sector remains on for only a portion of each sampling interval. The duration of each such portion for each sector of the fan beam is determined by the feedback control, on the basis of detecting the post-object beam for the same sector only or for other sectors as well. The shutter pins can be moved between their blocking and open positions by pin drivers similar in construction to the pin driver heads of dot matrix printers, which in turn are controlled through a feedback loop as a function of the radiation detected at a number of angular positions in the fan beam emerging from the object. In this embodiment the shutter pins are not constrained to precise end positions. They are required to move rapidly between their end positions but the end positions need not be precisely fixed so long as in its blocking position a shutter pin substantially completely interrupts its sector of the fan beam and in its open position a shutter pin substantially completely uncovers its sector of the fan beam. The shutter pins can be in a single row across the width of the fan beam or in multiple rows. While their cross-section can have almost any shape, round pins are convenient to manufacture and use, and can be arranged in a staggered array, to overlap partly such that each beam sector controlled by a shutter pin overlaps partly with at least one adjacent sector. The feedback detector can be a linear array of detector elements extending along the width of the fan beam and moving so as to follow the sweeping motion of the fan beam, or can be a detector having a large enough detection area to receive the emerging fan beam at all times without moving or with only limited motion. In each case, the feedback detector can be in front of the image plane or behind the image plane. Of course, if it is in front of the image plane, it should be reasonably transparent to x-ray so as not to attenuate the imaging beam excessively, and its x-ray transmission characteristics should be reasonably uniform across the detection area so as not to introduce excessive mask artifacts in the image. In an alternative embodiment the modulation scheme can still be called pulse width modulation but differs in that in any one sampling time interval a shutter pin can block and uncover its sector of the fan beam two or more times. The goal still is to make the on times of the sector add up in a sampling interval to a cumulative time which is just enough for a suitable exposure level. In this case each shutter pin can be a monostable device driven by an impulse away from its stable positon against a bias force and driven back when the bias force overcomes the pin inertia. The stable position can be either when the pin is in its blocking position or in its open position. In a still alternate embodiment, beam width (or, more accurately, thickness) modulation can be added by making controlling each shutter pin such that in its open position it does not uncover its sector of the beam completely but continues to block a portion of its sector to, in efect, reduce the thickness of its sector by a controlled amount in each respective sampling time interval.
In a particular exemplary but nonlimiting embodiment of the invention, an x-ray source provides a fan beam of penetrating radiation and a scanning mechanism sweeps the fan beam across the object which is to be imaged in a sweeping direction which is transverse to the plane of the fan beam. An imager receives the fan beam which emerges from the object after suffering attenuation due to its passage therethrough and uses this emerging fan beam to form an image of the swept object. A feedback system also receives the emerging fan beam and generates a feedback signal as a function thereof, for example as a function of the radiation detected at each of a number of angular positions in the emerging fan beam. A shutter mechanism individually pulse width modulates sectors of the fan beam as a function of the feedback signal so as to reduce variations in a selected parameter of the radiation in the emerging beam received by the imager and to thereby improve a selected parameter of the image. Pulse width modulation in this context refers to dividing the sweeping motion into a number of sampling time intervals and keeping each sector on for only a part of each sampling interval, the duration of the on time in each sampling interval for each sector of the fan beam being determined as a function of the feedback signal. Alternative embodiments of the invention can combine such pulse width modulation of the sectors of a fan beam with beam width modulation of the sectors. In this context beam width modulation refers to dynamically changing the cross-sections of respective sectors of the fan beam, while the beam sweeps the object, as a function of the feedback signal. One way to do this in accordance with the invention is to change the feedback scheme such that in its open position a shutter pin need not completely uncover its sector of the fan and to precisely control the open positions of the pins so as to control the cross-section of the sectors during the on times in the sampling time intervals. In each case, the goal of the modulation scheme is to reduce variations in one or more parameters of the emerging beam received by the imager so as to improve the quality of the image formed thereby. It is desirable to establish an appropriate relationship between the dimensions of the slit collimator which defines the fan beam thickness, the speed of this collimator and the duration of a sampling time interval. Preferably, this relationship is such that the duration of a sampling time interval is an integral submultiple of the time T.sub.s that the slit collimator takes to move through the slit dimension which defines the fan beam thickness, e.g., to have 2 or 3 or 4 sampling time intervals per time T.sub.s but not a non-integral multiple such as 2.2 or 3.4 sampling time interval per T.sub.s. In addition, it is desirable to synchronize the sampling time intervals with the waveform of the line voltage which powers the x-ray tube. For example, if the line voltage is at 60 Hz the number of sampling time intervals per sweep per sector of fan beam 12 can be 15, 30, 60 or 120, and the start of a time interval can be synchronized with a cycle of the line voltage waveform in a way which reduces undesirable effects of the ripple on the waveform of the power supplied to the x-ray tube. In one embodiment of the invention there can be a respective feedback detector element for each shutter pin, such that a feedback channel is made up of a shutter pin, the beam sector controlled by that pin and the feedback detector element on which that sector impinges (and the feedback network to process the detector element output and drive the pin). However, in alternate embodiments additional benefits can ensue from greater freedom in the use of the feedback detector output to control the off and on times of the beam sectors. For example, if there are shutter pins 1, 2, 3, . . . , n, . . . , N, where n is a positive integer, and there is a fan beam sector and a feedback detector element for each pin, the on and off time for fan beam sector n in a given sampling time interval can be determined by only the output of detector element n, or by the output of several detector elements, for example by a selected combinations of detector elements n-M through n+M, where M is an integer. For example, the on and off times of beam sector n can be controlled as a function of the combination C of the outputs of three detector elements with appropriate weighting, i.e., by C=[0.25(n-1)+0.5n+0.25(n+1)]. The feedback signal for an angular position in the emerging fan beam can be derived as a function of the exposure measured at that angular position and a sector of the fan beam can therefore be turned off as soon as enough exposure has been measured at its angular position or positions in a given sampling time interval, with some accounting for feedback lag time. In the alternative, the feedback signal from each angular position for an initial part of a sampling time interval can be used to estimate how long it would take to reach a desired exposure level at each respective angular position at the feedback detector plane. The latter technique allows outputs for several angular positions at the feedback detector plane to be combined effectively, without inaccuracies due to differences in the closure times of different shutter pins. In the alternative the latter technique can be used to reduce the effect of lag time, particularly in a case where the lag time of the feedback system is high, for example because of high transit times in the movement of pins between their end positions.
One of the advantages of the invented pulse width modulation scheme of individual sectors of a sweeping fan beam is that it can achieve highly effective and practical exposure equalization and thereby can advantageously enhance desirable qualities of the image and suppress undesirable qualities. In conceptual terms, the overall effect of exposure equalization in this context could be viewed as a way to enhance the high spatial frequencies in the image and suppress the low spatial frequencies, or as an edge enhancement. In a simplistic way, this could be viewed as a result similar to that of blurring an image, for example by dividing the image into areas and replacing the pixel values in each area by the average of the pixel values in that area, and then producing a new image by subtracting on pixel-by-pixel basis the blurred image from the original image. In practical terms, the invention achieves accurate control over the relevant image qualities which is flexible enough to allow different desired levels and kinds of image qualities to be enhanced or suppressed and which adequately takes into account the practical limitations of the components and systems that are used to implement the invention. For example, the feedback in the invented system is handled in a way which removes undesirable effects of the inherent lag in feedback channels which involve mechanical motion of components such as the shutter pin. Moreover, the feedback scheme is such that it can respond rapidly enough to the inherent time variations in the output of the x-ray tube, in a way which does not detract from image fidelity. Other object and advantages of the invention will become apparent from the detailed discussion below taken in conjunction with the drawings.